Effect of five insecticides on microbial and enzymatic activities in sandy soil

Abstract

A laboratory study was conducted to examine the effects of five insecticides on microbial and enzymatic activities important to fertility in sandy soil. Cyfluthrin significantly increased bacterial populations after 2 wks. Imidacloprid showed an inhibitory effect on fungal numbers after 2 wks incubation while the others did not affect fungal population. No inhibitory effect was observed on nitrification of soil indigenous nitrogen. All treatments stimulated S‐oxidation after 4 wks. With the exception of cyfluthrin and imidacloprid after 2 wks, denitrification in sandy soil indicated that all treatment inhibited denitrification throughout the experiment. No inhibitory effects on biomass‐c were observed during 2‐wk periods. An inhibitory effect was observed on amylase after 1 wk while significant recovery was observed after 3 wks. With the exception of HgCl₂, no effect was observed on reducing sugar formation for 2 wks with all treatments. Formazan formation resulting from dehydrogenase activity was significantly greater with tebupirimphos and Aztec for 1 wk. All treatments supressed phosphatase activity for 1 wk, while none of the treatments suppressed phosphatase activity after 2 wks. Amitraz, tebupirimphos and Aztec inhibited urease activity for 1 wk. With the exception of tebupirimphos, no treatments affected N₂‐fixation in soil. Although short‐lived inhibitory effects on activities of microbes and enzymes were caused by the insecticides, the soil indigenous microbes can tolerate the chemicals used for control of soil pests.     

Alterations of microbial populations and composition in the rhizosphere and bulk soil as affected by residual acetochlor

Acetochlor is a widely used herbicide in maize fields; however, the ecological risk of its residue in the soil–plant system remains unknown. We investigated the dissipation dynamics of field dose acetochlor and clarified its impact on microbial biomass and community structure both in the rhizosphere and bulk soil over 1 month after its application. Soil microbial parameters such as quantities of culturable bacteria and fungi represented by colony-forming units, soil microbial biomass carbon (SMB_C), and phospholipid fatty acids (PLFAs) were determined across different sampling times. The results showed that the dissipation half-lives of acetochlor were, respectively, 2.8 and 3.4 days in the rhizosphere and bulk soil, and 0.02–0.07 μg/g residual acetochlor could be detected in the soil 40 days after its application. Compared to the bulk soil, microbial communities in the rhizosphere soil were inclined to be affected by the application of acetochlor: SMB_C content and bacterial growth were most likely to be increased; however, fungal growth was prone to be inhibited. The principal component analysis of PLFAs, as well as the comparisons of fungi/bacteria and cy17:0/C16:1ω9c ratios between different treatments over sampling time, revealed that the soil microbial community composition was significantly affected by acetochlor at its early application stage (at day 15); thereafter, the effects of acetochlor were attenuated or even could not be detected. Our results suggested that residual acetochlor did not confer a long-term impairment on viable bacterial groups in the rhizosphere and bulk soil.     

Microbial community structure in a silty clay loam soil after fumigation with three broad spectrum fungicides

The short-term effect of three broad spectrum fungicides on microbial activity, microbial biomass, soil ergosterol content, and phospholipid fatty acid (PLFA) profiles was studied. A silty clay loam soil was treated separately with captan, chlorothalonil and carbendazim at three different dosages of each fungicide. Chlorothalonil and carbendazim significantly altered soil microbial activity. However, changes in soil microbial biomass were only observed in soil treated with higher dosages of these fungicides. All dosages of fungicides significantly decreased fungal biomass as estimated by soil ergosterol content. PLFA analysis indicated that there was a shift in PLFA pattern. Higher dosages of all three fungicides decreased a straight-chain PLFA 22:0. In addition, soil treated with carbendazim increased cyclopropyl fatty acids. Compared to untreated soil, higher dosages of both captan and chlorothalonil affected PLFA 10Me 16:0, indicating that these fungicides can reduce actinomycetes population. Finally, our results suggest that application of both captan and chlorothalonil decreased Gram-positive to Gram-negative ratio.     

Bioremediation of Cd and carbendazim co-contaminated soil by Cd-hyperaccumulator Sedum alfredii associated with carbendazim-degrading bacterial strains

The objective of this study was to develop a bioremediation strategy for cadmium (Cd) and carbendazim co-contaminated soil using a hyperaccumulator plant (Sedum alfredii) combined with carbendazim-degrading bacterial strains (Bacillus subtilis, Paracoccus sp., Flavobacterium and Pseudomonas sp.). A pot experiment was conducted under greenhouse conditions for 180 days with S. alfredii and/or carbendazim-degrading strains grown in soil artificially polluted with two levels of contaminants (low level, 1 mg kg^?1 Cd and 21 mg kg^?1 carbendazim; high level, 6 mg kg^?1 Cd and 117 mg kg^?1 carbendazim). Cd removal efficiencies were 32.3–35.1 % and 7.8–8.2 % for the low and high contaminant level, respectively. Inoculation with carbendazim-degrading bacterial strains significantly (P?<?0.05) increased Cd removal efficiencies at the low level. The carbendazim removal efficiencies increased by 32.1–42.5 % by the association of S. alfredii with carbendazim-degrading bacterial strains, as compared to control, regardless of contaminant level. Cultivation with S. alfredii and inoculation of carbendazim-degrading bacterial strains increased soil microbial biomass, dehydrogenase activities and microbial diversities by 46.2–121.3 %, 64.2–143.4 %, and 2.4–24.7 %, respectively. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analysis revealed that S. alfredii stimulated the activities of Flavobacteria and Bradyrhizobiaceae. The association of S. alfredii with carbendazim-degrading bacterial strains enhanced the degradation of carbendazim by changing microbial activity and community structure in the soil. The results demonstrated that association of S. alfredii with carbendazim-degrading bacterial strains is promising for remediation of Cd and carbendazim co-contaminated soil.     

Total bacterial and fungal population after chlorpyrifos and quinalphos treatments in groundnut (Arachis hypogaea L.) soil

Short-term inhibitory effect on the total bacterial population was observed after chlorpyrifos and quinalphos applications in the groundnut fields, which recovered within 60 days after seed treatment and by 45 days of soil treatment. The fungal population was significantly enhanced after chlorpyrifos treatment whereas quinalphos inhibited the fungal population during the initial days of treatment but no effect was observed after 60 days of treatment. The residues of chlorpyrifos and quinalphos in the treated soil were not persistent and their half-lives ranged from 7.0 to 9.2 days and 13.2 to 20.6 days, respectively.     

Adsorption, transformation, and bioavailability of the fungicides carbendazim and iprodione in soil, alone and in combination

When studying the effect of mixtures of toxic substances on soil organisms, attention must be paid to peculiarities in exposure to mixtures as opposed to that of single toxicants. The fungicides carbendazim and iprodione compete in the adsorption to soil. The presence of iprodione reduced the adsorption of carbendazim by 30%, while carbendazim reduced the adsorption of iprodione by 70%. Iprodione had little effect on the transformation rate of carbendazim in soil. However, carbendazim retarded the transformation of iprodione in soil by 26%. The concentration of the fungicides in pore water was found to be substantially higher for mixtures than when a fungicide alone was present in the soil. The effect of the additional fungicide on the concentration is especially apparent in the period following the first 1 to 2 weeks of the incubation. The inclusion of copper in the mixture has little additional effect on the concentration of the fungicides in pore water.     

Influence of different abiotic and biotic factors on the metalaxyl and carbofuran dissipation

Metalaxyl and carbofuran dissipation was studied in response to different factors (soil bacterial communities, light irradiation, presence of an inorganic culture medium and presence of soil) and combinations of these factors in short-term experiments (48 h). The soil microbial communities have no effect on metalaxyl or carbofuran dissipation in the time scale employed. Light irradiation and soil promote metalaxyl and carbofuran dissipation by photodegradation and adsorption, respectively. However, photodegradation has a stronger effect on metalaxyl and carbofuran dissipation than the adsorption of the pesticides in the soil. The addition of the culture medium have no direct effect on pesticide dissipation, degradation by microbial communities or adsorption but its presence greatly increased photodegradation.     

Structural changes in soil communities after triclopyr application in soils invaded by Acacia dealbata Link

Triclopyr is a commonly used herbicide in the control of woody plants and can exhibit toxic effects to soil microorganisms. However, the impact on soils invaded by plant exotics has not yet been addressed. Here, we present the results of an 18-month field study conducted to evaluate the impact of triclopyr on the structure of fungal and bacterial communities in soils invaded by Acacia dealbata Link, through the use of denature gradient gel electrophoresis. After triclopyr application, analyses of bacterial fingerprints suggested a change in the structure of the soil bacterial community, whereas the structure of the soil fungal community remained unaltered. Bacterial density and F:B ratio values changed across the year but were not altered due to herbicide spraying. On the contrary, fungal diversity was increased in plots sprayed with triclopyr 5 months after the first application. Richness and diversity (H´) of both bacteria and fungi were not modified after triclopyr application.     

Chloride concentration affects soil microbial community

We studied the effect of increased inorganic chloride concentration on forest soil microflora in a laboratory experiment. Microbial DNA extracted from experimental soil samples was amplified with PCR using primer pairs specifically amplifying bacterial, eukaryotic and fungal DNA fragments. The resulting amplified DNA was further used for terminal restriction fragment length polymorphism (TRFLP) analysis. Our work revealed that chloride concentration affects the indigenous microbial community in experimental soil. This was documented on an unidentified microorganism whose DNA was detectable in soil high in chloride but was not found in soil with low chloride concentration. The presence of the organism responsive to increased chloride concentration was associated with the highest observed value of chlorination of humic acid, suggesting possible role of this organism in soil chlorine turnover. High chloride concentration in the soil tended to decrease the rate of degradation of trichloroacetic acid. The problems connected with measurement of chlorination rates in soil are discussed.     

Pesticide influence on soil enzymatic activities.

The influence of four pesticides, e.g. glyphosate, paraquat, atrazine, and carbaryl, on the activities of invertase, urease and phosphatase of twenty-two soils, numbered as 1-22, was investigated. Soils displayed a general variability of enzyme activities with invertase being more abundant than urease and phosphatase in the order listed. The addition of glyphosate and paraquat activated invertase and urease activities in several soils. Increments of invertase activity ranged from a very low increase (+4%) up to +204% in soils 11 and 14, respectively. Smaller increases were measured for urease. A general inhibitory effect (from 5% to 98%) was observed for phosphatase in the presence of glyphosate. The effects of atrazine and carbaryl on the three soil enzymes were evaluated against that exhibited by methanol, the solvent used for their solubilization. In almost all soils, atrazine further inhibited invertase activity with respect to the inhibitory effect shown by methanol. By contrast, consistent activation effects (from 61% to 10217%) were measured for urease with methanol alone and/or methanol-pesticide mixtures. Contradictory results were observed with phosphatase. Similarities found between the results obtained with enzymes in soils and those measured with synthetic enzyme complexes (e.g. free enzymes and/or clay-, organo-, and organo-clay-enzyme complexes) exposed to the same pesticides allowed some relationships between responses of soil enzymes to pesticides and soil properties to be hypothesized.     

外源Cd对不同利用方式红壤脲酶活性的影响

采用室内培养实验,通过将外源Cd添加到同一母质、全镉含量相近的不同利用方式的红壤中(林地、水稻土和菜园土),研究了外源Cd污染对不同利用方式红壤脲酶活性的影响。结果表明:整个培养过程中,Cd污染对3种红壤脲酶活性都有抑制作用,且随重金属浓度的增强而增强。同剂量Cd污染对3种红壤脲酶活性的抑制效应不同,大小为林地>水稻土>菜园土。实验设定的Cd处理水平下,对林地、水稻土和菜园土脲酶活性产生显著抑制作用(p<0.05)的Cd浓度分别为5、30和50 mg/kg土。     

Degradation of juvenile hormone analog by soil microbial isolates

Juvenoids are efficient pesticides with relatively low toxicity to humans. However, few studies have evaluated the effect of degradation by soil microorganisms on their toxicity. The effects of bacterial, fungal and yeast isolates on aerobic decomposition of ethyl N-[2-[4-(2,2-ethylenedioxy-1-cyclohexylmethyl)phenoxy]ethyl] carbamate during eight weeks were determined. The effect of different concentration of glucose on their degradation activity is also analyzed.     

Effects of pesticides on activities of enzymes and microorganisms in a clay soil

Laboratory experiments were conducted to determine the effect of 32 pesticides applied at 2 levels on populations of microorganisms, activities of urease, dehydrogenase, phosphatase and nitrogenase in a clay loam incubated for 1 week. Results indicated that a decrease in bacterial number was observed with thiram for 2 days and stimulation with chlorpyrifos after 7 days. Some fungicides and fumigants inhibited fungal numbers for 2 days. The recovery was rapid and stimulatory effects on microbial numbers were evident in many samples. None of the pesticides inhibited soil urease drastically. Formazan formation was not suppressed vigorously by the treatments. With the exception of DD and Vorlex at a high level, none of the treatments inhibited phosphatase in the hydrolysis of p-nitrophenyl disodium orthophosphate. A temporary decrease in nitrogenase activity in acetylene (C2H2) reduction was observed with many pesticides. The low amount of pesticides applied to the clay loam is unlikely to have detrimental effects on soil microbes and the enzymes important to soil fertility.     

Impacts of organic carbon availability and recipient bacteria characteristics on the potential for TOL plasmid genetic bioaugmentation in soil slurries

The effectiveness of genetic bioaugmentation relies on efficient plasmid transfer between donor and recipient cells as well as the plasmid’s phenotype in the recipient cell. In the present study, the effects of varying organic carbon substrates, initial recipient-to-donor cell density ratios, and mixtures of known recipient bacterial strains on the conjugation and function of a TOL plasmid were tested in sterile soil slurry batch reactors. The presence of soil organic carbon was sufficient in ensuring TOL plasmid transconjugant occurrence (up to 2.1 ± 0.5%) for most recipient strains in soil slurry batch mating experiments. The addition of glucose had limited effects on transconjugant occurrence; however, glucose amendment increased the specific toluene degradation rates of some Enterobacteriaceae transconjugants in soil slurry. Initial cell density ratios and mixtures of recipient strains had smaller impacts on plasmid conjugation and resulting phenotype functionality. These observations suggest that genetic bioaugmentation may be improved by minimal altering of environmental conditions.     

Effects of sulfonamide and tetracycline antibiotics on soil microbial activity and microbial biomass

Increasingly often soil residual concentrations of pharmaceutical antibiotics are detected, while their ecotoxic relevance is scarcely known. Thus, dose related effects of two antibiotics, sulfapyridine and oxytetracycline, on microorganisms of two different topsoils were investigated. The fumigation-extracted microbial C (E(C)) and ergosterol were determined to indicate soil microbial and fungal biomass, respectively. Microbial activity was tested as basal respiration (BR), dehydrogenase activity (DHA), substrate-induced respiration (SIR), and Fe(III) reduction. The BR and DHA were uninfluenced even at antibiotic concentrations of 1000 microg g(-1). This revealed that an activation of microbial growth through nutrient substrate addition is required to test possible effects of the bacteriostatic antibiotics. In addition, the effects of both antibiotics were time dependent, showing that short-term tests were not suitable. Clear dose-response relations were determined with SIR when the short-term incubation of 4h was extended into the growth phase of the microorganisms (24 and 48 h). The Fe(III) reduction test, with a 7-d incubation, was also found to be suitable for toxicity testing of antibiotics in soils. Effective doses inhibiting the microbial activity by 10% (ED(10)) ranged from total antibiotic concentrations of 0.003-7.35 microg g(-1), depending on the antibiotic compound and its soil adsorption. Effective solution concentrations (EC(10)), calculated from distribution coefficients, ranged from 0.2 to 160 ng g(-1). The antibiotics significantly (p<0.05) reduced numbers of soil bacteria, resulting in dose related shifts in the fungal:bacterial ratio, which increased during 14 d, as determined from analysis of ergosterol and E(C). It was concluded that pharmaceutical antibiotics can exert a temporary selective pressure on soil microorganisms even at environmentally relevant concentrations.     

Effect of hydrocarbon pollution on the microbial properties of a sandy and a clay soil

The aim of this work was to ascertain the effects of different types of hydrocarbon pollution on soil microbial properties and the influence of a soil's characteristics on these effects. For this, toxicity bioassays and microbiological and biochemical parameters were studied in two soils (one sandy and one clayey) contaminated at a loading rate of 5% and 10% with three types of hydrocarbon (diesel oil, gasoline and crude petroleum) differing in their volatilisation potential and toxic substance content. Soils were maintained under controlled conditions (50-70% water holding capacity, and room temperature) for six months and several microbiological and toxicity parameters were monitored 1, 60, 120 and 180 days after contamination. The toxic effects of hydrocarbon contamination were greater in the sandy soil. Hydrocarbons inhibited microbial biomass, the greatest negative effect being observed in the gasoline-polluted sandy soil. In both soils crude petroleum and diesel oil contamination increased microbial respiration, while gasoline had little effect on this parameter, especially in the sandy soil. In general, gasoline had the highest inhibitory effect on the hydrolase activities involved in N, P or C cycles in both soils. All contaminants inhibited hydrolase activities in the sandy soil, while in the clayey soil diesel oil stimulated enzyme activity, particularly at the higher concentration. In both soils, a phytotoxic effect on barley and ryegrass seed germination was observed in the contaminated soils, particularly in those contaminated with diesel or petroleum.     

Impact of coexistence of carbendazim, atrazine, and imidacloprid on their adsorption, desorption, and mobility in soil

The effect of coexisting pesticide on adsorption/desorption and mobility of another one was investigated with carbendazim (CBD), imidacloprid (IDP), and atrazine (ATR). The data indicated that adsorption of CBD, ATR, and IDP on the tested soil was fitted well by Freundlich equation and increased with an order of IDP < ATR ? CBD. Adsorption of a pesticide was decreased by the coexistence of another one through their competitive adsorption. The presence of coexisting solute of the more adsorbability played a more important role than that of the lesser adsorbability. The adsorption of IDP and ATR was easier to be affected by 28.9–52.0 % and 31.1–60.7 % with the addition of CBD, while that of CBD was much less influenced by 3.4–18.1 % and 6.9–31.8 % with the presence of ATR and IDP, respectively. An adsorbability-related enhancement in desorption of the three pesticides by the co-adsorbed solute was also observed. As a result of competitive adsorption/desorption, the mobility of the pesticides estimated from soil thin-layer chromatography was altered. The results clearly illustrated that adsorbability and concentration-related alteration in adsorption/desorption and mobility will be caused by the coexistence of pesticides.     

Bacterial, azotobacter, actinomycetes, and fungal population in soil after diazinon, imidacloprid, and lindane treatments in groundnut (Arachis hypogaea L.) fields

Bacterial, azotobacter, actinomycetes, and fungal populations were determined in groundnut (Arachis hypogaea L.) fields between July and November for three consecutive years (1997-1999) after insecticide treatments. Diazinon was applied for both seed and soil treatments. However, imidacloprid and lindane were used for seed treatments. An average half-life (t1/2) of diazinon in seed- and soil-treated fields was found to be 29.32 and 34.87 days, respectively. Its residues were found for 60 days in both cases. In diazinon seed treatment, an increase in azotobacter, fungi, and actinomycetes populations was observed in samples from the 15th and 30th days, and this trend continued until crop harvest. However, the bacterial population had not been affected by this treatment. The diazinon soil treatment had indicated some significant adverse effects on fungi and actinomycetes population, which recovered after 30 days. The population of bacteria and azotobacter increased significantly in this treatment. The residues of imidacloprid and lindane were found for 90 and 120 days with an average half-life of 40.9 and 53.3 days, respectively. Imidacloprid had no significant effect on fungi and actinomycetes populations up to 15 days, and between 15 to 60 days some adverse effects were indicated. However, some significant increases in bacterial and azotobacter population were observed. Lindane had no effect on bacterial and fungal population. However, its adverse effects were observed in actinomycetes and azotobacter populations between 30 to 60 days.     

Restoring biochemical activity and bacterial diversity in a trichloroethylene-contaminated soil: the reclamation effect of vermicomposted olive wastes

Background, aim, and scope  In this work, the potential for using olive-mill solid waste as an organic amendment for biochemical and biological restoration of a trichloroethylene-contaminated soil, which has previously been stabilized through vermicomposting processes, has been explored. Materials and methods  Trichloroethylene-contaminated water was pumped into soil columns with a layer of vermicompost at 10-cm depth (biobarrier system). The impacts of the trichloroethylene on the microbial community were evaluated by determining: (1) the overall microbial activity (estimated as dehydrogenase activity) and enzyme activities related to the main nutrient cycles (β-glucosidase, o-diphenoloxidase, phosphatase, urease, and arylsulphatase activities). In addition, isoelectric focusing of the soil extracellular humic-β-glucosidase complexes was performed to study the enzymatically active humic matter related to the soil carbon cycle. (2) The soil bacterial diversity and the molecular mechanisms for the bacterial resistance to organic solvents were also determined. For this, polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis (DGGE) was used to detect changes in bacterial community structure and PCR-single-strand conformational polymorphism (SSCP) was developed and optimised for detection and discrimination of the resistance-nodulation-division (RND) genes amplified from the contaminated soils. Results  Vermicompost reduced, with respect to the unamended soil, about 30% of the trichloroethylene leaching during the first month of the experiment. Trichloroethylene had a marked negative effect on soil dehydrogenase, β-glucosidase, urease, phosphatase, and arylsulphatase activities. Nevertheless, the vermicompost tended to avoid this toxic effect. Vermicompost also displays stable humic-β-glucosidase complexes that increased the extracellular activity related to C-cycle in the contaminated soils. The isoelectric focusing technique showed a more biochemically active humic matter in the soil sampled under the vermicompost. The behaviour of the three main phyla of bacteria isolated from the DGGE bands was quite different. Bands corresponding to Actinobacteria disappeared, whereas those affiliated with Proteobacteria remained after the trichloroethylene contamination. The disappeared Actinobacteria became visible in the soil amended with the vermicompost. Bands corresponding to Bacteriodetes appeared only in columns of contaminated soils. In this study, six types of RND proteins were detected by PCR-SSCP in the natural soil, three in the trichloroethylene-contaminated soil and 7/5 in trichloroethylene-contaminated soil above/below the vermicompost in the biobarrier columns. Trichloroethylene tended to reduce or eliminate all the clones detected in the uncontaminated soil, whereas new efflux pumps appeared in the biobarrier columns. Discussion  Although enzymes incorporated into the humic substances of vermicomposted olive wastes are quite stable, trichloroethylene also inhibited the background levels of the soil extracellular β-glucosidase activity in the amended soils. The decrease was less severe in the biobarrier system, but in any case, no relation was found between the levels of trichloroethylene in soil and extracellular β-glucosidase activity, or between the latter and the quantity of humic carbon in soils. The isoelectric focusing technique was carried out in the humic fraction to determine whether the loss of activity occurred in overall extracellular β-glucosidase or in that linked to stable humic substances (humic–enzyme complexes). The contaminated soils showed the lower enzyme activities, whereas contaminated and amended soils presented greater quantity of focalised (and therefore stable) humic carbon and spectra heterogeneity: very different bands with higher enzyme activities. No clear relationship between trichloroethylene concentration in soil and diversity of the bacterial population was noted. Similar patterns could be found when the community structures of bacteria and microbial activity were considered. Since the use of the dehydrogenase assay has been recognised as a useful indicator of the overall measure of the intensity of microbial metabolism, these results could be attributed to PCR-DGGE methodology, since the method reveals the presence of dominant populations regardless of their metabolic state. Trichloroethylene maintained or even increased the number of clones with the DNA encoding for RND proteins, except for the contaminated soil located above the vermicompost. However, the main effect of trichloroethylene was to modify the structure of the community in contaminated soils, considering the type of efflux pumps encoded by the DNA extracted from soil bacteria. Conclusions  Trichloroethylene inhibited specific functions in soil and had a clear influence on the structure of the autochthonous bacterial community. The organic matter released by the vermicomposted olive waste tended to avoid the toxic effect of the contaminant. Trichloroethylene also inhibited the background levels of the soil extracellular β-glucosidase activity, even when vermicompost was present. In this case, the effect of the vermicompost was to provide and/or to stimulate the humic-β-glucosidase complexes located in the soil humic fraction >10⁴, increasing the resistance of the enzyme to the inhibition. The bacterial community from the soil presented significantly different mechanisms to resistance to solvents (RND proteins) under trichloroethylene conditions. The effect of the vermicompost was to induce these mechanisms in the autochthonous bacterial community and/or incorporated new bacterial species, able to grow in a trichloroethylene-contaminated ambient. Coupled biochemical and molecular methodologies are therefore helpful approaches in assessing the effect of an organic amendment on the biochemical and biological restoration of a trichloroethylene-contaminated soil. Recommendations and perspectives  Since the main biochemical and biological effects of the organic amendment on the contaminated soil seem to be the incorporation of biochemically active humic matter, as well as new bacterial species able to grow in a trichloroethylene-contaminated ambient, isoelectric focusing and PCR-SSCP methodologies should be considered as parts of an integrated approach to determine the success of a restoration scheme.     

Seasonal variations in sugar contents and microbial community in a ryegrass soil

The relationship among sugar concentrations, microbial community and physical variables (precipitation and soil temperature) was investigated in a ryegrass soil from January 2004 to January 2005. Mono- and disaccharide sugars were extracted using a mixture of dichloromethane and methanol and analyzed as their TMS derivatives by GC-MS. Changes in microbial community were assessed using phospholipid and neutral lipid fatty acids (PLFA and NLFA, respectively) analysis. The results of a one-year study showed that the seasonal variability of sugar contents found in the soil samples is mainly related to biomass or nutritional status of the fungal community. The increase in sucrose and fructose exudation by plant roots in the beginning of the growing season (early spring) may be responsible for the highest fungal biomass amount (PLFAs) observed in this study. Fungal storage lipid abundances (NLFAs) peaked in summer, during the same period that the highest concentrations of mannitol and trehalose were detected. This is consistent with these two sugars being stress-induced fungal metabolites, produced due to the low soil moisture observed during this season. In contrast, bacterial community growth seems to be more dependent on plant substrate than on physical variables, since the strongest decrease in bacterial biomass amounts (PLFAs) was found after cutting of the ryegrass field in early July.